1. Field of the Invention
The present invention relates in general to a system and a method for protecting a torque converter of an automotive automatic transmission, and more particularly to a system and a method for preventing overheat of the torque converter.
2. Description of the Prior Art
In torque converter automatic transmissions of a motor vehicle, engine power is inputted through a torque converter to a transmission where the power is subjected to a path change (or gear change) for meeting each particular driving situation of the vehicle. One example of this type transmission is described in NISSAN CAR MANUAL tiled "Service Manual for RE4R01A type Automatic Transmission" issued from Nissan Motor Co., Ltd. in March 1987.
The torque converter and the transmission are operated under common usage of a hydraulic fluid. Since the torque conversion by the torque converter is carried out through the hydraulic fluid, the fluid tends to have a higher temperature. This tendency becomes marked as the load of the engine is increased, and thus when a higher load operation of the engine is continued for a longer time, the hydraulic fluid in the torque converter becomes to have a very high temperature.
In the torque converters of a lock-up type wherein input and outlet elements can be directly engaged through a lock-up clutch, such a high temperature of the hydraulic fluid tends to induce a premature deterioration of the facing material of the lock-up clutch and thus lower the durability of the automatic transmission.
In order to deal with such undesired phenomenon, various measures have been proposed and put into practical use. Almost all of them are of a type in which, as is described in the above-mentioned NISSAN CAR MANUAL and Laid-open Japanese Patent Application (Tokkai) 8-42660, when the temperature of the hydraulic fluid in the torque converter is detected or judged higher than a critical level, a certain operation is carried out to lower the heat generated by the torque converter.